1. Field of the Invention
The invention relates to optical disc drives, and more particularly to the servo systems of optical disc drives.
2. Description of the Related Art
A servo system of an optical disc drive controls operations of the pickup head and the disc motor to read optical discs of different formats in different conditions. A plurality of servo signals is used to respectively control different aspects of the servo system. For example, a tracking error signal controls tracking of the servo system, and a focusing error signal controls focus of the servo system. The servo signals are synthesized according to the reflection signals of a main-beam and a sub-beam, both laser beams projected onto the surface of the optical disc by the pickup head. To obtain reliable servo signals in different operating conditions of the optical disc drive, the servo signals require appropriate gain compensation in the synthesis thereof, and a mechanism for precisely controlling the servo signal gains is thus required.
FIG. 44 is a schematic diagram of the focused spots of a main-beam and two sub-beams incident on the surface of an optical disc. Data is recorded on the grooves of the surface of the optical disc in the form of pits or spaces. When a focused spot of a laser beam is projected on the groove, the reflection intensity of the laser beam is determined according to whether a pit or a space is located at focused spot, and data is thus read according to the reflection intensity. The focused spot 7102 of the main-beam is projected on the groove N by the pickup head and moves along the groove N to read data recorded thereon, and the focused spots 7104 and 7106 of the two sub-beams are respectively projected on the lands N and N−1 next to the groove N. The reflection intensities of the main-beam and the sub-beams are then detected by photo-detectors of the pickup head. The servo signals are synthesized according to the reflection intensities, and the servo system is then controlled according to the servo signals to improve data reading.
However, when the reflectivity of a read portion of the optical disc surface is changed, the signal strengths detected by the photo-detectors also change, affecting the reliability of synthesized servo signals. For example, the reflectivity of the blank zone on which no data is recorded is different from the reflectivity of the data zone on which some data is recorded. If the read portion of the optical disc is switched from the blank zone to the data zone, the signal strength levels of the servo signals will vary due to the reflectivity difference. Thus, amplification adjustment of the servo signals is required to automatically compensate the effect of the reflectivity difference, to maintain servo signal strength at the same level in different reflectivity conditions.
Additionally, if the intensity of a main-beam incident to the optical disc is changed, amplification adjustment of the servo signals may be also required. FIG. 45 is a schematic diagram contrasting the reflectivity of the optical disc surface to the intensity of the main-beam. When the intensity of the main-beam is increased, the reflection intensity of the main-beam received by photo-detectors declines because the surface of the groove melts due to the raised intensity of the main-beam. When the groove surface melts, data is written to the surface of the groove. However, the reflection intensity of the sub-beam remains at the same level. Because some of the servo signals are synthesized according to both reflection intensities of the main-beam and the sub-beam, the amplification adjustment of the servo signals is also required to balance the difference between the reflection intensities of the main-beam and the sub-beam, to maintain the servo signal strength at the same level.